1. Field of the Invention
The present invention relates generally to a charging apparatus of rechargeable batteries built into electronic devices such as mobile terminals, and, more particularly, to a charging IC (Integrated Circuit), charging apparatus and an electronic device using as a charging source of the rechargeable batteries an external power source such as an AC (Alternating Current) adaptor or a USB (Universal Serial Bus) host such as a personal computer connected with a USB cable.
2. Description of the Related Art
In charging of a rechargeable battery built into an electronic device such as a mobile terminal, which uses an AC adaptor or a personal computer as a charging source, preliminary charging, constant-current charging or constant-voltage charging is selected depending on a voltage level of the rechargeable battery, and even in the constant-current charging, a current value may be gradually switched.
In regard to such charging of a rechargeable battery, known propositions are a charging circuit having a drooping characteristic depending on a output current (For example, Japanese Patent Application Laid-Open Publication No. 1999-327671 (paragraph number 0020, FIG. 1, FIG. 2 and the like)), a method and apparatus used for constant-voltage charging after constant-current charging for charging a battery of a cellular phone (For example, Japanese Patent Application Laid-Open Publication No. 2003-274570 (paragraph number 0016, FIG. 4, FIG. 5 and the like))
By the way, a description is made for the charging which switches to constant-voltage charging after constant-current charging with an external power source having a drooping characteristic. FIG. 1 is a circuit diagram of a charging apparatus using a PMOS (P-channel Metal Oxide Semiconductor) transistor for charging control; FIG. 2 is a flowchart of the charging control; and FIG. 3 is a diagram showing a charging operation.
This charging apparatus 2 is configured to use an AC adaptor 8 as a charging source to charge a rechargeable battery 6 built into an electronic device 3 along with an apparatus load 4. The charging apparatus 2 is provided with a charging IC 10, and the charging IC 10 integrates circuit units such as a charging control unit which can be integrated into an IC. As an external circuit of the charging IC 10, a charging path 11 is formed for passing a charging current between the AC adaptor 8 and the rechargeable battery 6; the charging path 11 is equipped with a PMOS transistor (Tr) 12 as a charging control element; and the PMOSTr 12 is connected serially with a backflow prevention diode 14 and a sense resistor 16. The sense resistor 16 converts the charging current flowing through the charging path 11 to a voltage to be detected and the detected current is applied to the charging IC 10 as control information. Control output obtained in the charging IC 10 is applied to a gate of the PMOSTr 12 to execute charging-current control or constant-voltage control.
In the charging of the rechargeable battery 6 with the charging apparatus 2, as shown in FIG. 2, when the AC adaptor 8 is detected as the charging source, the output voltage of the AC adaptor 8 is detected, and then, a battery status check is performed for a battery temperature and a battery voltage which indicate a battery status (step S1). If the result of the battery status check is normal, a charging operation is started. At the start of the charging operation, first constant-current charging CC1 is performed as fast charging (step S2), except the case that a battery voltage is too low (an over-discharge state), and the battery voltage is raised by the first constant-current charging CC1 to a start voltage of constant-voltage charging (step S3). After the battery voltage is raised, second constant-current charging CC2 is performed (step S4), and when full charging is achieved through constant-voltage charging (step S5), the charging is completed. To a constant current Icc1 of CC1, a constant current Icc2 of CC2 is in a relationship of Icc2<Icc1.
In this charging process, as shown in FIG. 3, after the rechargeable battery 6 is raised to a predetermined voltage (e.g., 3 [V]) through preliminary charging (PR) with a preliminary current Ipre, a charging current Ic is increased to a constant current Icc1 to utilize the drooping characteristic of the AC adaptor 8 to perform the first constant-current charging CC1 and second constant-current charging CC2, and after constant charging CV, the charging of the rechargeable battery 6 is reaches to, for example, 4.2 [V] and completed. FIG. 3 is a diagram showing transitions of charging forms, charging currents and charging voltages. In this case, the output voltage of the AC adaptor 8 is set higher than the charging completion voltage, and maximum heat generation happens at the point of time when the second constant-current charging is switched to the constant-voltage charging.
The heat generation amount due to the charging operation is described with reference to FIG. 4. FIG. 4 is a diagram showing transitions of the heat generation amount in each portion during the charging operation.
In the charging operation, for the heat generation in the PMOSTr 12, diode 14 and sense resistor 16 in the first constant-current charging CC1 region (t1 to t2), the heat generation of the diode 14 is largest and accounts for one-half of the total heat generation amount. In the second constant-current charging CC2 region (t2 to t3), the total heat generation amount is larger than the constant-current charging CC1 region (t1 to t2), and the heat generation amounts of the diode 14 and sense resistor 16 is smaller than the CC1 region (t1 to t2), while the heat generation amounts of the PMOSTr 12 accounts for two third of the total heat generation amount. In the constant-voltage charging CV region (t3 to t4), the heat generation amount is reduced dependently on reduction of the charging current. In the total heat generation amount of the region (t1 to t4), the heat generation amount of the PMOSTr 12 is largest. In this heat generation, when making the shift to the constant-current charging CC2, PMOSTr 12 generates heat depending on electric power applying a voltage. Such heat generation leads to energy loss.
By the way, a start voltage of the constant-current charging CC1 is set to, for example, about 3 [V], and the charging current Ic is dependent on the output of the AC adaptor 8 because the drooping characteristic of the AC adaptor 8 is used. Therefore, in the CC1 region, the charging current Ic is on the order of 670 [mA]. Assuming that an output voltage of the AC adaptor 8 is Voa and a battery voltage is Vb, the output voltage Voa is as follows:Voa=Vb+(external part resistance [Ω]×Ic)  (1)
The external part generates heat depending on this acceptable loss.
In this case, assuming that a voltage drop of the diode 14 is 0.3 [V], the heat generation amount Pd is as follows:Pd=0.3 [V]×Ic  (2)
When a constant current Icc2 in the CC2 region is set to, for example, 350 [mA], if the output voltage Voa is about 5.4 [V], a large portion of a difference voltage ΔV (=Voa−Vb) between the battery voltage Vb and the output voltage Voa of the AC adaptor 8 is concentrated on the PMOStr 12, and the heat generation amount of the PMOStr 12 accounts for a large portion of the total heat generation amount. Assuming that the loss due to the external part is about 456 [mW], about 318 [mW] is lost by the heat generation of the PMOSTr 12. Since the charging current Ic comes down below CC1 at the time of the shift to CC2, the battery voltage Vb comes down to on the order of 3.9 [V] and then, the battery voltage Vb is raised by the constant-current charging CC2 to make the shift to the constant-voltage charging CV at the battery voltage Vb=4.2 [V].
In the constant-voltage charging CV, constant-voltage control is performed such that the battery voltage Vb will be 4.2 [V], and as the rechargeable battery 6 approaches full charge, the charging current Ic is reduced. In the CV region, since the charging current is small, the heat generation is lower than the CC2. The charging is completed and the charging operation is terminated if the charging current Ic reduced to on the order of 50 [mA].
A charging apparatus 2 in FIG. 5 is a case that a PMOSTr 18 is used as a backflow prevention element, instead of the diode 14 described above. In FIG. 5, the same symbols are added to the same portions as FIG. 1. In the charging apparatus 2, the PMOSTr 18 is operated by a control signal applied from the charging IC 10 to a gate to regulate the direction of the charging current Ic passing through the charging path 11.
In regard to the heat generation amount of the charging apparatus 2 shown in FIG. 5, as shown in FIG. 6, in the first constant-current charging CC1 region (t1 to t2) the heat generation of the sense resistor 16 is largest and accounts for one-half of the total heat generation amount. The total heat generation amount in the second constant-current charging CC2 region (t2 to t3) is about twice larger than the constant-current charging CC1 region (t1 to t2), and the heat generation amounts of the PMOSTr 18 and the sense resistor 16 are dropped to on the order of one-fifth of the total heat generation amount, while the heat generation amount of the PMOSTr 12 accounts for four-fifth of the total heat generation amount. In the constant-voltage charging CV region (t3 to t4), the heat generation amount is reduced dependently on reduction of the charging current. In this heat generation, when making the shift to the constant-current charging CC2, PMOSTr 12 generates heat depending on electric power applying a voltage. As described above, such heat generation leads to energy loss.
In a charging apparatus 2 shown in FIG. 7, a switching power source 20 is disposed with in the charging IC 10, and the output of the switching power source 20 is picked up after passing though a filter circuit 26 consisting of an inductor 22 and capacitor 24 and is supplied to the rechargeable battery 6 through the charging path 11. In this case, although the PMOSTr 18 for backflow prevention is disposed in the charging path 11, the PMOSTr 18 may be omitted.
In the charging apparatus 2, the AC adaptor 8 is used as a charging source, and the output of the AC adaptor 8 is applied to the switching power source 20 to generate constant currents Icc1, Icc2 and a constant voltage Vc which achieve a first constant-current charging CC1, second constant-current charging CC2 and constant-voltage charging CV. Therefore, in regard to the heat generation amount of the charging apparatus 2 (FIG. 7), as shown in FIG. 8, in the first constant-current charging CC1 region (t1 to t2) the heat generation of the switching power source 20 is largest and accounts for one-half of the total heat generation amount. The total heat generation amount in the constant-current charging CC2 region (t2 to t3) is reduced to on the order of one-half of the constant-current charging CC1 region (t1 to t2); the heat generation amount of the switching power source 20 also comes down to on the order of one-half of the total heat generation amount as compared to the region of the constant-current charging CC1 (t1 to t2); and in the constant-voltage charging CV region (t3 to t4), the heat generation amount is reduced dependently on reduction of the charging current.
By using such a switching power source 20, when the charging current Ic of the constant-current charging CC1 is enlarged, the heat generation amount is increased, and although the capacity of the switching power source 20 may be increased in order to constrain the heat generation, the cost is increased. In other words, if a DC-DC conversion efficiency ηof the switching power source 20 is η=90 [%], the heat generation amount Ph is Ph=about 347 [mW] and the heat generation amount is increased. By increasing the output current of the switching power source 20 to on the order of 700 [mA] in order to ensure the charging current Ic, the cost is increased.
Therefore, in regard to electronic devices, such as cellular phones, equipped with rechargeable batteries, device chassis are miniaturized and thinned as well as equipped batteries are planned to be large capacity, and it is requested to constrain the heat generation when the devices are operated for phone calls and the like during the charging operation.
Japanese Patent Application Laid-Open Publication Nos. 1999-327671 and 2003-274570 do not disclose such issues and not disclose or indicate any configurations for solving the issues.